Temperature conversion calculator

The formula

$$°\text{F} = \tfrac{9}{5}\,°\text{C} + 32$$ $$°\text{C} = \tfrac{5}{9}\,(°\text{F} - 32)$$ $$\text{K} = °\text{C} + 273.15$$

Where it comes from

Temperature scales are built from two fixed points. On both the Celsius and Fahrenheit scales, the reference points are water freezing and boiling at sea-level pressure:

	Freezes	Boils	Gap
Celsius	0	100	100
Fahrenheit	32	212	180

The same physical gap is 100 Celsius degrees or 180 Fahrenheit degrees. So one Celsius degree is $180/100 = 9/5$ Fahrenheit degrees. To convert, scale the distance above freezing by $9/5$, then add back Fahrenheit’s freezing point of 32.

Kelvin uses the Celsius degree size but starts at absolute zero, the point where particles have the least possible thermal energy. That’s −273.15 °C, so adding 273.15 converts Celsius to kelvin. Kelvin has no degree sign, and no negative values.

Worked example

72 °F, a comfortable room:

$$°\text{C} = \tfrac{5}{9}(72 - 32) = 22.2\ °\text{C}$$ $$\text{K} = 22.2 + 273.15 = 295.4\ \text{K}$$

Mental shortcuts

• °C to °F, roughly: double it and add 30. 20 °C gives about 70 °F (actual 68).
• −40 is the same on both scales. It’s the one temperature where you don’t need to convert.

Watch out for

• Temperature differences convert without the +32. A 10 °C rise is an 18 °F rise.
• Gas laws and other physics formulas need kelvin, not °C.

Sources

• NIST Office of Weights and Measures, "SI Units: Temperature"

Last checked November 11, 2026